The present invention relates to a high-hardness silica glass and a method of producing it, and more particularly to high-hardness silica glass as transparent as usual silica glass and having a Vickers hardness of 12-20 GPa useful for optical parts such as optical elements, optical communication elements, etc., structural materials such as fibrous materials added to composite materials, and window glasses for watches, aircrafts, autoclaves and other high-pressure containers.
Pure silica glass is highly excellent in transmittancy and also has a relatively high hardness (Vickers hardness of 5.8-6.9 GPa). The addition of some additives to further increase its hardness, however, usually leads to the deterioration of transmittancy thereof.
Other types of glass provided with improved hardness are reportedly crystallized glass, oxinitride glass, chemically reinforced glass, etc. Among them, what has the highest hardness is the oxinitride glass which contains nitrogen in the glass structure to increase its hardness. Why the glass is provided with higher hardness by introducing nitrogen into the glass structure is not necessarily clear, but it may be presumed that the formation of Si--N bonds in the glass structure makes the glass denser.
The oxinitride glass may be produced by various methods as shown below:
(1) Blowing an NH.sub.3 gas into a molten glass heated at high temperatures.
(2) Treating porous glass with an NH.sub.3 gas at high temperatures.
(3) Using a nitride starting material and melting it at high temperatures.
(4) Using a nitride starting material and melting it at high temperatures and pressure.
Actually, however, any of the above methods can hardly provide glass having excellent transmittancy. Some types of oxinitride glass having high transmittancy and hardness which have been reported so far are shown in Table 1.
TABLE 1 ______________________________________ Composition N(at %) Vickers Hardness (GPa) ______________________________________ Y-Al-Si-O-N 5 11.67 La-Si-O-N 18 11.96 ______________________________________
As shown in Table 1, considerable improvement in hardness has been achieved, but further improvement in hardness is desired for glass used in severe applications. In addition, strong demand exists to provide a method of easily producing glass having high transmittancy and hardness.